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Glutes—once the province of spandex-clad women clenching in the aerobics studio or lunging along to their VHS copies of Buns of Steel—are switching sides. Whether it's a flabby ass in need of firming or a flat one that could use some inflation, the backside is the newest trend in men's fitness.

"Beautiful Booty is a class that was designed for women, but now we're seeing more and more men sign up," says Stephanie Vitorino of an Equinox in Woodland Hills, California, the creator of the VBody DVD series. "The look that men were previously gravitating toward—what I call the light-bulb look, with big arms and a huge chest that tapers down to skinny legs—is no longer attractive," Vitorino says. "Men want to be more balanced."

This new approach still relies, in part, on the glute exercises of yore—squats and lunges—but the emphasis is now on combining more traditional routines with explosive movements and muscle shredders like plyometrics and cardio. "I don't do squats as much anymore," says New York-based personal trainer and author David Kirsch, who was dubbed the Master of Ass by Victoria's Secret model Karolina Kurkova and who has helped sculpt the derrieres of Anne Hathaway, Heidi Klum, and Liv Tyler. "Running stairs, doing lateral bench step-ups, cycling—they're all very effective for working the glutes without being as tough on your joints."

Like defined abs and bulked-up calves, flattering glutes can be elusive. "Genetics are genetics—if you're predisposed to having a flat ass, it's going to be more difficult to meet your goal, but it's not impossible," Kirsch says. "Guys, for the most part, can be prone to building muscle, and they can really get their asses jacked."

Not that looks are everything. Conditioning your glutes also offers less visible benefits, like easing lower-back pain and improving posture. "The bottom line," says Vitorino, "is that men are beginning to realize their bottom line is important."

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3 WAYS TO GET BETTER GLUTES

Celebrity trainer David Kirsch, author of The Butt Book (available at amazon.com), offers three moves you can use to build your glutes before ever stepping in the gym. Try to do three sets of 10 to 15 reps.

Isometric Contractions
Whether you're sitting at your desk or stuck in traffic, you can work on your glutes using isometric contractions. Simply contract your butt muscles, hold, and relax. Repeat.

The Platypus Walk
Start in a plié-squat position (knees spread wide) with your hands behind your head, your thighs parallel to the ground, and your feet parallel to your thighs. Waddle forward for five steps and then reverse. Keep your knees out and your weight back.

Frog Jumps
Start in a squat position with your feet wider than your shoulders. Place your hands behind your head and jump up, then return to a squat. Repeat.